The Bosch Automotive Handbook/Bosch 23rd updated and expanded edition, Braunschweig; Wiesbaden, Vieweg, 1999, ISBN 3-528-03876-4 page 115 describes an ultrasonic flow measuring system. This system allows the propagation time t of an acoustic pulse as it travels through a medium to be measured (e.g., air) at an angle of inclination α. One measurement is taken upstream and one downstream using the same measuring path 1. The resulting transit time differential is proportional to the volumetric flow rate.
In this document, see page 115, right-hand column, figure, a flow channel is described in whose walls two sensors facing one another are situated. The faces from which the acoustic pulses are emitted face one another.
Furthermore, ultrasonic flow sensors are known from the related art, which use the beam drift effect within a flowing medium for measuring the flow velocity. Furthermore, ultrasonic transducers manufactured using micromechanical or film technology are known from the related art.